1. Field of the Invention
The present invention relates to a secondary suspension agent for suspension polymerization of a vinyl compound, a suspension stabilizing agent for suspension polymerization of a vinyl compound, and a process for producing a vinyl polymer. More particularly, it pertains to a secondary suspension agent and a suspension stabilizing agent each excellent in the capability of exhibiting a high dispersibility even by a small usage and further of rendering vinyl polymer particles porous in the case of suspension polymerization of a vinyl compound such as vinyl chloride for the production of the vinyl polymer, and a process for producing a vinyl polymer using the agents.
2. Description of the Related Arts
In industrially producing a vinyl polymer such as a vinyl chloride resin, there has heretofore been widely practiced a suspension polymerization process in which a vinyl compound such as vinyl chloride monomer is suspended in an aqueous medium in the presence of a suspension stabilizing agent and polymerized by the use of an oil-soluble catalyst. In general, the factors governing the quality of the vinyl polymer to be produced thereby include conversion to polymer, water to monomer ratio, polymerization temperature, kind and amount of catalyst, type of polymerization vessel, agitation rate, kind and amount of the suspension stabilizing agent and the like. It is known that among them, the kind of the suspension stabilizing agent produces a great influence on the aforesaid quality.
The performance required for a suspension stabilizing agent for suspension polymerization of a vinyl compound include that (i) the agent functions so as to exhibit a high dispersibility and make the granulometric distribution of the vinyl chloride polymer granules obtained as sharp as possible by the use of a small amount of the agent; (ii) the agent functions so as to make the polymer granules obtained as uniform and porous as possible to increase the absorption rate of a plasticizer for facilitating the workability and to facilitate the removal of the vinyl chloride monomer residue in the polymer granules to prevent the formation of fish eye, etc. in the molding to be obtained; (iii) the agent functions so as to produce polymer granules having a high bulk density; and the like.
As a suspension stabilizing agent for suspension polymerization of a vinyl compound, there has heretofore been employed a cellulose derivative such as menthylcellulose and carboxymethylcellulose, partially hydrolyzed polyvinyl alcohol or the like, alone or in combination. For example, Japanese Patent Application Laid-Open No. 115890/1977 discloses a process for suspension polymerization of vinyl chloride using as the secondary suspending agent, a partially hydrolyzed polyvinyl alcohol having a saponification value of 300 to 500 (equivalent to a degree of hydrolysis of 37.3 to 69.7 mol%) and an average molecular weight of 10,000 to 30,000 (equivalent to an average degree of polymerization of 140 to 530), and Japanese Patent Application Laid-Open No. 112210/1980 discloses a process for suspension polymerization of vinyl chloride using as the secondary suspending agent, a partially hydrolyzed polyvinyl alcohol having a saponification value of 505 to 600 (equivalent to a degree of hydrolysis of 14.6 to 36.3 mol%) and a viscosity of 0.5 to 2.0 cPs as measured in 4% solution thereof in methanol (equivalent to an average molecular weight of 550 or less). However, the above-mentioned secondary suspending agents have suffered the vital disadvantage that the bulk density of the vinyl chloride polymer granules thus obtained is lowered though some effect is recongnized on the porosity thereof.
In addition, Japanese Patent Application Laid-Open No. 6392/1978 discloses a process for suspension polymerization of vinyl chloride using as the secondary suspending agent, a partially hydrolyzed polyvinyl alcohol having a degree of hydrolysis of 40 to 55 mol% and a viscosity of 5 to 15 mPas as measured in 4% solution thereof in mixed solvents of isopropanol and water (1:1 by mixing ratio) (equivalent to an average molecular weight of 74 to 780). However, the aforesaid secondary suspending agent has suffered the drawback of low capability of making porous the vinyl chloride polymer granules obtained, low absorption rate of plasticizer, or difficulty in removing the vinyl monomer residue.
Moreover, Japanese Patent Application Laid-Open No. 67319/1990 discloses a suspending agent for suspension polymerization of vinyl chloride which comprises a polyvinyl alcohol having a degree of hydrolysis of 80 mol% and an average degree of polymerization of 3000 and a polyvinyl alcohol having a degree of hydrolysis of 40 mol% and an average degree of polymerization of 3000. Japanese Patent Application Laid-Open No. 167745/1981 discloses an aqueous dispersion comprising as the suspending agent, a polyvinyl alcohol having a degree of hydrolysis of 65 mol% or more and an average degree of polymerization of 100 to 3000 and, as the dispersoid, a polyvinyl ester having a degree of hydrolysis of 20 to 65 mol% and an average degree of polymerization of 1000 or less, which aqueous dispersion is usable as the secondary suspending agent for suspension polymerization of vinyl chloride. However, the use of any of the suspending agents as disclosed above can not sufficiently solve the above-stated problems such as low porosity and low bulk density of the vinyl chloride polymer granules.
Further, WO91/15518 discloses the use of an aqueous dispersion comprising as the dispersoid, a polyvinyl ester polymer having an end modified with an ionic group which polyvinyl ester having a degree of hydrolysis of 60 mol% or less and an average degree of polymerization of 50 to 3000 as the secondary suspending agent for suspension polymerization of a vinyl monomer such as vinyl chloride. Nevertheless, a number of difficult problems, especially the low bulk density of the vinyl chloride polymer to be produced remain unsolved in the use of the aforesaid agent.
Furthermore, Japanese Patent Application Laid-Open No. 110797/1977 discloses a secondary suspending agent for suspension polymerization of vinyl chloride which comprises a partially hydrolyzed polyvinyl alcohol having a degree of hydrolysis of 30 to 65 mol% and a degree of polymerization of 60 to 6,000. However, the secondary suspending agent has suffered the defects of low bulk density of the vinyl chloride polymer granules obtained and low absorption rate of plasticizer in spite of some improvement in the porosity thereof.